Etymology
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sill (n.)

Old English syll "beam, threshold, large timber serving as a foundation of a wall," from Proto-Germanic *suljo (source also of Old Norse svill, Swedish syll, Danish syld "framework of a building," Middle Low German sull, Old High German swelli, German Schwelle "sill"), perhaps from PIE root *swel- (3) "post, board" (source also of Greek selma "beam"). Meaning "lower horizontal part of a window opening" is recorded from early 15c.

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Definitions of sill

sill (n.)
structural member consisting of a continuous horizontal timber forming the lowest member of a framework or supporting structure;
sill (n.)
(geology) a flat (usually horizontal) mass of igneous rock between two layers of older sedimentary rock;
From wordnet.princeton.edu